The words whirled, world sound the same but have different meanings and spellings. Why do whirled, world sound the same even though they are completely different words?
The answer is simple: whirled, world are homophones of the English language.
To revolve rapidly about a center or an axis. See Synonyms at turn.
To rotate or spin rapidly: The dancer whirled across the stage.
To turn rapidly, changing direction; wheel: She whirled around to face him.
To have the sensation of spinning; reel: My head is whirling with data.
The earth with its inhabitants.
The inhabitants of the earth; the human race.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").